Botany was the first Apologia Elementary we used, and my 6th grade dd did it mostly on her own, with help from us for the experiements. She loved it so much that she is this year, in 8th grade, doing botany again with a college-level textbook; we started out this year with the only upper-level botany course for homeschoolers that we could find, but she got frustrated with it and said she had learned more with the Apologia! Any homeschooler who brings it up in conversation gets an earful from her about how much she learned and enjoyed it.
When it was time to start my younger dd, I chose Zoology I because she was already interested in birds and flight, and we're doing Astronomy on the side (she's 9). She will do botany, too, but I'm going to wait until she's a bit older. Of the ones we've done, I think botany is the most advanced, maybe because most of the info was new to me so it seemed like it had the most to learn for the first time and retain. A friend who has an environmental ecology degree and specialized in botany looked at the Apologia course and said it contained some info she hadn't learned until college!
We really do love the Apologia elementary science courses!